This invention relates to unloaders for reciprocating compressors and more particularly to seals for sealing the stem of a compressor unloader.
Unloaders are used in conjunction with a reciprocating compressor to prevent the compressor cylinder pressure from increasing or to limit the magnitude of cylinder pressure increase during the compression stroke of the compressor piston in the compressor cylinder. When the unloader is in the closed position, the compressor functions as a conventional compressor. In this condition, it is operating in the normal or "loads" condition where the compressor cylinder pressure increases during the compression stroke of the compressor piston in the compressor cylinder. When the unloader is in the on position, the compressor cylinder gas is allowed to move to an adjacent chamber and bypass the compression cycle. When operating in this condition, the compressor is referred to as "unloads".
Unloaders provide operational flexibility in controlling compressor capacity. They can also be used to maintain operating pressures, temperatures, driver horsepower loads and piston rod loads within acceptable limits as operating conditions change. During start-up, they can reduce the load on the driver by the compressor cylinder so that available energy can be used to overcome rotational inertia, rather than for compression.
There are various types of unloaders such as plug unloaders which can be used with bypass or volume bottle unloader assemblies. Plug unloaders may be of the balanced or unbalanced type.
Plug unloaders are used with a suction unloader valve to deactivate the cylinder end; with either a suction or discharge unloader valve and volume bottle to add fixed clearance; or as a head end pocket unloader to add fixed clearance. The balanced plug unloader is designed to use the energy of the compressed gas to assist actuation. This minimizes the force required to load, allowing low pressure plant air to be used for actuation.
The plug style unloader uses a pneumatically actuated plug assembly to open and close a center port of a specially designed pressure valve. Air supply, provided through either a remote or integral control valve, acts inside a piston and cylinder chamber to deliver a smooth, positive force to the stem and plug assembly. The balanced plug design uses a stem/plug assembly that moves along a fixed bonnet guide. Drilled holes in the base of the plug allow the gas pressure to remain below (balanced) the plug seat and between the bonnet guide and plug (balanced) as the plug opens and closes. Actuator forces are therefore reduced so smaller, less costly actuator designs can be used.
When the plug is in the closed position, the unloader valve functions as a conventional compressor valve; however, when the plug is in the open position, compressor cylinder gas is allowed to move to the adjacent suction or discharge compression chamber and bypass the compression cycle.
When used with a volume bottle and in the open position, compressor cylinder gas expands inside a fixed clearance bottle during each compression stroke. When used as a head end unloader, the result is similar. In this case, the plug opens and closes an integral cavity of the compressor cylinder end.
Unbalanced (solid plug) unloaders operate similarly to that of the balanced unloaders except that the plug is solid and does not include drilled holes in the base of the plug as does the balanced unloader. Thus, actuator forces are greater than that of the balanced unloader since the gas pressure is fully applied to the plug and not allowed to pass through the holes in the plug as in the balanced unloader.
The various types of unloaders include a reciprocating stem passing through a bonnet and having one end affixed to the plug. The stem reciprocates within the bonnet as the plug moves between the loaded and unloaded positions of the unloader. Thus, dynamic seals must be provided to seal between the reciprocating stem and bonnet. Various types of unloader stem dynamic seals have been used including elastomeric, polymeric, or a combination of elastomeric-polymeric seals which are both pressure energized and mechanically energized.
The leakage of environmentally unsafe gasses into the atmosphere is of great concern to the process industry. In particular, fugitive emissions of process gasses may escape into the atmosphere through leaks in reciprocating compressors. If a major leak occurs, the plant may have to be shut down until the leakage is corrected.
The EPA requirements for these dynamic stem seals are rigorous and do not allow emissions around the unloader stem. Prior art compressor valve unloaders leak process gas to the atmosphere past the unloader stem dynamic seal or seals. This leakage can be a major problem when the process gas is toxic or hazardous.
The present invention overcomes the deficiencies of prior art dynamic seals for unloader stems.